I had a doppler ultrasound a year ago, upon my request, at my cardiologists office being I only have one side with blood flow since the left internal and external carotid arteries were removed during cancer surgery, which came back with no blockage.

I have read literature that suggests this test maybe should be done yearly, 3 or 5 years after treatment, when stenosis may begin, and risk increases as we age, and the further we are out from radiation, and depending on where you were radiated, amount of radiation, and other risks, and some maybe sooner that are at higher risk for artherosclerosis who smoke, have diabetes, high cholesterol, high blood pressure and such.

That's my brief understanding, and guess it's something to discuss with your health team, but here is the OCF page on Carotid Artery Stenosis that explains it better.

http://www.oralcancerfoundation.org/complications/carotid-artery-stenosis.php